Electric submersible pumps

ABSTRACT

A method of pumping wellbore fluid, comprising the steps of: installing an electric submersible pump in a wellbore; and operating the pump at more than 4,500 rpm to pump the wellbore fluid. Pumping in this manner provides a number of advantages in use in that the required high-speed motor and pump is shorter for a given power than existing arrangements, and provides increased reliability due to reduced complexity. A much shorter motor/pump combination also allows such equipment to be used in deviated boreholes with a reduction in damage due to mishandling and bending, as well as facilitating assembly and testing in the manufacturer&#39;s plant.

This invention relates to motors and electronic drives for electricsubmersible pumps and compressors, and is concerned more, but notexclusively, with centrifugal pumps.

Submersible pumping is a well-established technique for extractinghydrocarbons from deep boreholes, where the natural pressure in thereservoir is insufficient to lift the fluid or gas to surface. Thetechnique is also used in water production.

Typically the production requirement is to lift large volumes of liquidagainst a pressure difference related to the depth of the well in whichthe pump is installed. For very heavy crude oils, slow-speed positivedisplacement pumps are suitable. These are usually rotated by a motor atthe surface connected to the pump by a long flexible rod system.Centrifugal pumps have been found most suitable for normal crude oils,gas and water. These pumps are rotated by a submerged motor connecteddirectly to the pump, with electric power being delivered from thesurface by a long cable. Also, the use of electric cables makesinstallation possible in deep or long horizontal wells which wouldotherwise not be possible with the use of rotating rods.

The electric motors used for driving the centrifugal pumps are veryelongated, sometimes of a length of more than one hundred times theirdiameter. The resulting complexity of such a device, the difficulty ofits manufacture and the quantity of the degradable insulation materialsit employs all reduce the system reliability.

Electric motor shaft power output is defined as the product of rotationspeed and torque. For a given physical size and type of motor there is alimit to the level of torque that can be produced, typically due toself-heating. A high-speed motor therefore provides a means forobtaining more power from the same length of motor, or the same powerfrom a shorter length.

The output of a pump is normally given in terms of its hydraulic power,which is the product of flow rate and lifting pressure (in rationalisedunits). Centrifugal pump technology is characterised by the power outputbeing proportional to the cube of the rotational speed. This knownrelationship, sometimes termed the “affinity laws” means that arelatively small increase in the rotational speed can give rise to asubstantial power increase.

Centrifugal pumps are frequently made with hundreds of impellersthreaded on a common shaft, each impeller adding a little to the liftingpressure. Reducing the number of impellers by increasing the speed wouldtherefore afford an improvement in reliability.

The above demonstrates that a high-speed motor and pump would, by beingshorter for a given power, present direct advantages in reliability dueto reduced complexity, or alternatively yield a higher output for asimilar size. A large proportion of boreholes are deviated from thevertical and commonly even to the horizontal. A much shorter motor/pumpcombination would also lead to a reduction in damage caused bymishandling and bending during deployment through the curved sections ofthe borehole. Furthermore, the much-shortened length would allowmotor/pump combinations to be assembled and tested in ideal conditionsat the manufacturer's plant prior to being transported to the boreholelocation.

As will be described more fully below, innate limitations in theestablished motor and motor controller technology used in the electricsubmersible pumping industry have prevented the objective of higherspeed being recognised or addressed.

Historically, electric submersible motors used for centrifugal pumpinghave been of the asynchronous, or induction, type. The stator is made ofsteel laminations and copper windings, and the rotor of steellaminations with copper bars forming the winding known as a squirrelcage. The rotor laminations are keyed to a shaft, this shaft providingthe means of transmitting output torque. The rotor poles are produced byinduction or transformer action between the stator and the rotor, usinga portion of the stator current. The stator, in addition, produces arotating stator field due to the alternating current in its windings.Since the transformer coupling to the rotor requires an alternatingfield in the rotor, the rotor must turn at a different (lower) speedthan synchronous speed, producing a so-called slip frequency forinduction. Electric submersible motors are made with two poles in orderto achieve the maximum rotating speed from a standard 60 Hz utilitysupply. This speed is typically 3500 rpm, slightly less than theunattainable synchronous speed of 60 Hz×1 pole pair×60 s/min=3600 rpm.

It has become common to use variable speed drives to power these motors,rather than direct connection to the utility supply. Variable speeddrives first convert utility AC power, typically at 60 Hz, to DC, andthen by electronic switching convert the DC to a variable frequencyalternating voltage. The use of a variable speed drive confersadvantages during starting when it can limit the motor current to a safelevel, and during production when it can be used to manage flow rates.The latter is important when the changing characteristics of a reservoirare considered over its producing life. Although variable speed drives,by creating an artificial supply of 70 Hz or more, can operate the motorat higher speed than when directly connected to the utility supply, thisis a limited capability. Firstly the elongated induction motor is notsuited to high-speed operations due to internal losses and smallmechanical clearances, and secondly at the medium voltages used (oftenseveral thousand volts rms) drive losses become very high. Performanceis generally limited up to 80 Hz or about 4500 rpm.

In order to maximise the induced rotor pole strength it is necessary tominimise the gap between the rotor and the stator. Unless very hot, theoil in the gap is sheared by the rotor turning yet remains in laminarflow. As a result the friction absorbs several percent of motor power.Motor efficiencies above 90% are sought, and this is an important sourceof loss in existing motors. The internal heating caused by these losses,and the copper losses in the squirrel cage, reduce motor life by agingthe insulation materials.

The small gap is also a cause of premature failure due to mechanicalcauses. The limited diameter of boreholes is a natural disadvantage toboth motors and pumps, and as a result their design is very elongated. Apump and induction motor assembly for producing 250 HP may be 20 meterslong. This slender assembly is difficult to handle and particularlysubject to damage when being deployed into deviated or horizontal wells,since small deflections of the motor housing can cause the rotor toimpact on the stator. Rotor vibration due to bearing wear or imbalancealso increases the chance of rotor impact.

The requirement for the rotor to be made of laminations and the limitedoverall motor diameter act together to constrain the diameter of theinner torque-carrying shaft. It is common practice, for example, tocouple two 250 HP motors of 5.62 inch diameter together so as to make alonger 500 HP motor. Shaft strength limitation prevents this beingincreased to 750 HP or 1000 HP.

To provide a high-speed electric pumping system, it is desirable toincrease the rotor clearance, and to reduce the internal sources ofpower loss that increase with speed. It is also necessary to use a drivetechnology which remains efficient at high speed and at the differentoperating voltage levels needed for different motor speeds requiredduring the life of the well.

A further requirement of any high or low speed electric submersiblesystem using variable speed drives is to minimise the deleteriouseffects of the electrical switching used to produce the alternatingoutput voltages. Switching events on the long cables used in submersiblecable propagate as wave fronts that reflect at connections and mostparticularly at the motor terminals. These reflections cause voltagetransients that can approach twice the original voltage, and hencedestroy insulation to earth. Commonly the motor voltage is presumed tobe proportionally distributed through the turns of the stator winding,and the inter-turn insulation is less than that of the winding to earth.However a wave front impinging on a motor terminal must travel throughthe winding turn by turn before settling to its final value. Thereforethere are short periods in which one turn of a winding carries the wavefront at full voltage and an adjacent turn is unexcited. This internalvoltage difference can exceed the inter-turn insulation rating, againcausing premature failure. Increasing the insulation level to overcomethese problems reduces the space available for the copper in the windingand also reduces the heat transfer from the copper, so that the motorspecification is reduced.

An associated consideration for transients is the interference caused todata transmission systems used to convey data from instrumentationlocated in the well bore.

The foregoing has emphasised high-speed centrifugal pumping systems.However the same principles of reliable motor performance, matchingefficient drives and circumventing the effects of transients on longcables are all applicable to positive displacement pumping systems.

Positive displacement pumps have a flow rate essentially determined by acharacteristic volume per revolution multiplied by rotation speed. Thetorque demand at the pump shaft is determined by the back-pressure ofthe fluid column being lifted. These pumps usually operate at low speedsof a few hundred revolutions per minute. Since shaft power is theproduct of rotation speed and torque, it follows that these pumps arealso characterised by extremely high torque demand. Where there is sandproduction with the pumped fluid and where the wells are deviated orhorizontal, the rod connection to the surface has a very short workinglife. In these cases it is desirable to use a downhole motor with thepositive displacement pump.

However, induction motors are inherently unsuited to low speed and hightorque (although variable speed drives have improved their capabilitiesin this regard). Thus current installations rely on a gearbox to matchthe normal motor running speed and torque to the pump characteristics.This is also problematic as it is extremely difficult to make a reliablehigh torque gearbox in the small borehole diameter, and it is alsoexpensive.

A motor having high torque at any speed including low speed is thereforepreferable.

It is an object of the invention to provide an efficient electricsubmersible pump, comprising a reliable electric submersible motorcapable of operating at low, medium and high speeds, and overcoming manyof the above-described disadvantages of existing motors.

It is a yet further objective of the present invention to provide a highpower electrical submersible pumping system of the order of half thelength of conventional equipment.

According to one aspect of the present invention, there is provided amethod of pumping wellbore liquid, comprising the steps of:

-   -   a) installing an electric submersible pump in a wellbore; and    -   b) operating the pump at more than 4,500 rpm to pump the        wellbore liquid.

It should be understood that references herein to “pumping of wellboreliquid” are intended to encompass within their scope the pumping ofmultiphase fluids, that is mixtures of water and/or oil and gas, as wellas the pumping of wellbore liquid in a multi-lateral drillingenvironment in which the pump is operated to draw the wellbore liquidfrom a plurality of lateral well bores into a central sump.

According to further aspect of the present invention, there is providedan electric submersible pump comprising a permanent magnet motor havinga rotor comprising a plurality of permanent magnets equiangularly spacedabout a central shaft, a plurality of tubular elements supporting thepermanent magnets spaced at different axial locations along the shaft, aretaining sleeve tightly fitted over the permanent magnets so as toretain the permanent magnets on the shaft, and a stator coaxial with therotor comprising a stack of laminations and radially spaced coils woundaround the stack.

The invention also provides a motor having a rotor comprising a carriersleeve mounted on a central shaft, and a stator coaxial with the rotorcomprising a stack of laminations and radially spaced coils wound aroundthe stack, wherein the carrier sleeve is a loose fit on the shaft and issupported on the shaft by support rings tightly engaging the shaft.

The invention also provides a permanent magnet motor having a rotorcomprising a carrier sleeve mounted on a central shaft and bearing aplurality of permanent magnets having axial ends, and a retention sleeveextending over the magnets and having at least one end turned in over atleast one stress-relieving radially outwardly extending abutment part onthe carrier sleeve abutting an adjacent axial end of the magnets toretain the magnets in position on the carrier sleeve without damagingthe axial end of the magnet.

The invention also provides a permanent magnet motor having an elongaterotor provided with elongate permanent magnet means extendingtherealong, and a stator coaxial with the rotor, wherein the permanentmagnet means incorporates axially laminated parts to reduce eddy currentlosses.

The invention also provides a motor having a rotor and a stator coaxialwith the rotor, wherein the rotor is mounted in a bearing, and one ofthe stator and the bearing is provided with resiliently biasedprojection means for engaging within receiving means provided on theother of the stator and the bearing to prevent relative rotationtherebetween when the rotor begins to rotate with respect to the statoron starting of the motor.

The invention also provides a motor having a rotor and a stator coaxialwith the rotor, wherein the stator is mounted in a housing, the statorbeing locked within the housing by an axial key engaging within an axialgroove in at least one of the stator and the housing to prevent thestator from turning relative to the housing.

For a better understanding of the present invention and in order to showhow the same may be carried into effect, reference will now be made, byway of example, to the accompanying drawings, in which:

FIG. 1 schematically illustrates an electric submersible pumping system;

FIG. 1A is a block diagram of a variable speed drive;

FIG. 2 illustrates an embodiment of a downhole motor in accordance withthe invention in cross-section;

FIG. 3 illustrates the embodiment of FIG. 2 in axial section;

FIG. 4 illustrates the means of assembly of an elongated motor;

FIG. 5 illustrates a possible construction of a motor stator in ahousing;

FIG. 6 illustrates an alternative construction of a motor stator in ahousing;

FIG. 7 illustrates a possible rotor journal bearing;

FIG. 8 illustrates a possible rotor assembly;

FIG. 9 illustrates a bearing that creates internal support pressure;

FIG. 10 illustrates a known electrical representation of a permanentmagnet synchronous motor;

FIG. 11 shows the electrical waveforms of an idealised permanent magnetsynchronous motor;

FIG. 12 shows a known electrical circuit diagram for the output stage ofa variable speed drive;

FIG. 13 shows the typical electromotive force of a trapezoidal-woundpermanent magnet synchronous motor;

FIG. 14 shows the electrical waveforms of an idealised permanent magnetsynchronous motor operated as a brushless DC motor;

FIG. 15 shows representative waveforms of the variable speed drive ofFIG. 12 for a motor when a variable output voltage or current isrequired;

FIG. 16 shows representative waveforms of the variable speed drive ofFIG. 12 for a motor when a variable output voltage or current isrequired, but with a low switching frequency;

FIG. 17 shows representative waveforms of the variable speed drive ofFIG. 12 incorporating practical switches when a high-speed motor isdriven and a variable output voltage or current is required;

FIG. 18 shows the known idealised characteristics of positivedisplacement pumps and centrifugal pumps, turbines and fans;

FIG. 19 shows an electrical circuit diagram providing efficient meansfor varying the speed of positive displacement pumps and centrifugalpumps, turbines and fans by varying the internal voltage of a variablespeed drive;

FIG. 20 illustrates a means of providing the supply voltages to avariable speed drive in accordance with the invention;

FIG. 21 illustrates the improvement in efficiency provided by a variablespeed drive in accordance with the invention;

FIG. 22 illustrates a known phasor diagram for the interpretation of theoperation of an idealised permanent magnet synchronous motor accordingto FIG. 10;

FIG. 23 illustrates a means of optimisation of the control of apermanent magnet synchronous motor by varying the variable speed driveoutput voltage in accordance with the invention;

FIG. 24 illustrates a means of rotor assembly of an elongated permanentmagnet motor in a final position;

FIG. 25 shows a means of rotor assembly of an elongated permanent magnetmotor in an intermediate position;

FIG. 26 shows a means of stator assembly of an elongated permanentmagnet motor;

FIG. 27 shows a stator bore cross-section;

FIG. 28 shows a mandrel cross-section suited to the manufacture of astator assembly of an elongated permanent magnet motor of the presentinvention;

FIG. 29 shows a bearing outer housing suitable for insertion in anelongated permanent magnet motor of the present invention;

FIGS. 30 and 31 show schematic end views of the stator assembly of amotor in accordance with the invention, FIG. 31A showing a detail withina slot of the assembly;

FIG. 32 schematically illustrates the assembly of such a statorassembly;

FIG. 33 schematically illustrates an improved pumping system accordingto the present invention;

FIG. 34 illustrates a motor for the pumping system of FIG. 33;

FIG. 35 shows a further electrical circuit diagram for the output stageof a variable speed drive;

FIG. 36 shows an axial key between the stator and the housing for use ina motor of the present invention;

FIG. 37 shows a bearing outer ring suitable for use in a motor of thepresent invention;

FIG. 38 shows a cross-section through a coil suitable for use in a motorof the present invention; and

FIGS. 39 a and 39 b illustrate a possible means of forming a coil insuch a motor.

With reference to FIG. 1, a representative installation of an electricsubmersible pump (ESP) is shown. A borehole 101 drilled in the earth issealed with respect to the earth from the surface to below a reservoir102 with casing 103. The casing 103 is perforated at 104 to allowreservoir fluid to enter the well. A pump 107 is provided to lift fluidfrom the well up tubing 105 to the surface. The tubing 105 is sealed tothe casing 103 by packing 106 so that the reservoir fluid must gothrough the pump to reach surface. A permanent magnet submersible motor108 (PMSM) is mounted beneath the pump 107. The connecting shaft of themotor 108 passes through a seal and pump thrust bearing assembly 109,often termed a ‘protector’. The pumped fluid passes over the motor 108before entering the pump 107 and thus provides a certain amount ofcooling of the motor 108.

A power cable 110 for the motor 108 is run up past the pump 107 andalongside the tubing 105 until it emerges at the surface wellhead andpasses to a variable speed drive 111. This drive 111 is powered by theutility supply 112 or a generator.

It will be appreciated that other configurations of the installation arepossible, such as mounting the pump below the motor, and taking thecable up the tubing or making it an integral part of the tubing.Arrangements, such as that disclosed in U.S. Pat. No. 6,000,915, whichaccommodate the pump concentrically within the motor bore will generallybe found to make poor use of the limited borehole cross section and arenot preferred.

FIG. 2 shows a cross-section of an embodiment of PMSM in accordance withthe invention comprising a central rotor and surrounding annular statorwithin a housing 202. The rotor has a central shaft 201 for transmittingthe output torque, and a plurality of magnetically permeable sleeves 203carrying permanent magnets 204. The sleeves 203 are torsionally lockedto the shaft 201 by keys, shrinkage or other means in the art. It ispreferable to make the sleeves 203 separate from the shaft 201 as shown,for reasons of mechanical stability, to facilitate assembly and topermit the optimum strength material for the shaft 201 to be chosenindependently of the sleeve material. The magnets 204 are preferably ofa samarium cobalt composition as this gives the best economicperformance at the temperatures commonly found in deep boreholes usedfor hydrocarbon production. Other materials such as neodymium iron boronmay be used in appropriate circumstances, or improved materials as theybecome available.

During high-speed rotation the magnets 204 experience considerablecentrifugal force, and the adhesive that bonds them to the sleeves 203may weaken with age. A retaining sleeve 205, preferably of metal,provides a durable means of retention. To avoid the use of materialswhich degrade during prolonged operation at high temperature, it ispreferable to make the sleeve 205 a tight fit by shrinking it on, ratherthan depending upon tape, adhesives and fillers. The sleeve 205 ispreferably of one piece although, for ease of assembly, several shorterrings may be fitted adjacent to each other if required. U.S. Pat. No.4,742,259 discloses a technique for fitting a sleeve with axialconstraint. This technique requires the fitting of end washers that arepressed to the shaft to locate them without using positive abutments todo so. In a preferred arrangement shown in FIG. 8 a, rings 422,preferably made of non-magnetic, non-conducting material, may be slidonto the rotor sleeve 203, coming up against abutments 424, and theretaining sleeve 205 over the magnets 204 may be rolled over the outerfaces of the rings 422, as at 425, thereby locking the whole assembly inplace axially without the need for adhesives. Variations on this lockingmethod are possible within the scope of the invention, such as deformingthe sleeve 205 with a punch into a detent on the outer surface of thering 422, or using a snap ring and groove as a shoulder in place ofmachined feature 424.

The assembly so far described is termed the rotor, and the length ofmotor delineated by a sleeve is termed a rotor stage. FIG. 3 shows inaxial section a single rotor stage.

The magnets 204 are circumferentially disposed about the sleeve 203, andalternately poled in an essentially radial direction to cause aspatially alternating magnetic flux to cross the clearance gap 209.Other magnet arrangements will be known to persons skilled in the art.The entire motor and hence the gap 209 are filled with a benign fluid,such as a highly refined mineral oil, to balance the inside of the motoragainst the external wellbore pressure.

Preferably the magnets 204 are plated, for example in a vapourdeposition process, with corrosion-resistant material such as aluminium,so that they may resist corrosion from any ingress of moisture into themotor or from other sources, and so that any small loose particles ofmagnet material will be sealed into the magnets and not come free tocirculate within the motor bearing system.

Contained within the tubular housing 202 is a stack of thin magneticallypermeable laminations 206 as may be seen more clearly in FIGS. 3 and 4.Insulated wire, preferably made of copper coated with high-integrityinsulation such as polyetheretherketone or polyimide materials, is woundthrough the slots 207, and looped back 214 at the ends of the laminationstack as part of the coil winding process. The wound lamination assemblyconstitutes the stator of the motor.

The PMSM motor constructed as described will have many desirablecharacteristics for submersible pumping, associated with the generalnature of permanent magnet motors. For example, by providing a rotorflux from permanent magnets, there is no need to energise the rotor,unlike the field winding that requires separate power in an inductionmotor. This reduces the motor current by the amount needed for rotormagnetisation, which therefore reduces the ohmic loss in the statorwindings and the power cable. It also eliminates the rotor cage windingand thus an internal source of heating. The copper within the stator isused only for the production of the rotating stator flux. The inherenttorque output for the motor, which is derived from a product of spaceutilisation, rotor flux and stator flux, is very high compared to aninduction motor. This torque is available at any speed.

Further aspects of the motor construction may be addressed to givereliable high-speed performance. Firstly, as mentioned above, a majorsource of inefficiency in induction motors is the frictional drag in thenecessarily small rotor-stator gap. In a normal mass-produced PMSM thegap is also kept small in order to economise on the amount of magnetmaterial required. However, if it is considered that permanentlymagnetic material is not itself significantly magnetically permeable,then for magnetic purposes the gap between the stator and the rotor isthat between the lamination tip 210 and the outer surface of the sleeve203. The mechanical clearance gap 209 is only a part of this. Thus, if,for example, the magnet thickness was 3 mm and the clearance gap wasincreased from 0.25 mm to 1.25 mm, or 500%, the magnetic gap would onlyhave increased 30%. With only a modest increase in the amount ofmagnetic material it is possible to purposely design the motor to ensurea sufficiently large mechanical clearance such that at high speed thefluid in the clearance is turbulent. Above 5400 rpm for a rotor ofdiameter more than 50 mm a gap greater than 1.25 mm is preferred forthis purpose. A designer may use the known Reynolds number theory toestimate the needed gap size for other operating conditions, fluids andmotor sizes. Although the friction loss is higher in turbulent flow thanin laminar flow, turbulent flow ensures much more effective heattransfer between the rotor and the stator, so reducing the maximuminternal temperature. At any speed the large clearance will reduce thelikelihood of mechanical damage to the rotor during installation causedby bending of the outer housing, and also provide a measure of toleranceto contaminant particles.

Furthermore, it will be found that the deliberately large gap reducesthe eddy current losses, and hence heating, induced in the retainingsleeve 205 and magnetic material according to their conductivity. Theselosses increase approximately as the square of rotation speed, butdiminish with distance from the lamination tips 210. The inter-magnetspaces 213 may be filled but, unless care is taken to seal the cavity,particles of filler may dislodge over time and damage the motor. Ifrequired the cavities may be left unfilled. This is made possible by thesleeve 205, since it presents a low drag rotating surface to theclearance gap while making an enclosure to trap the fluid in thecavities. This trapped fluid is limited to bodily rotation or axial flowand does not contribute to friction in the clearance gap.

A further reduction in eddy current losses in the rotor can be obtainedby laminating the magnets axially. Rotor eddy current losses originatefrom flux harmonics in the stator, the eddy currents circulating on theface of the magnets and penetrating through the depth of the magnets andthen into the steel that the magnets are bonded to. Most of the ohmiclosses resulting from this current flow are in the magnets, assuming theretaining sleeve is non-conducting or very thin, and the current flowincreases with the face area of the magnets. Accordingly, in the sameway that the stator steel is laminated to reduce the effect of rotorflux, the magnets can be laminated to reduce the effect of stator flux.For an elongated motor, the face area is the width of the magnet timesthe continuous length of the magnet section. Therefore, by using aseries of short magnets to make a continuous length that areelectrically isolated from one another where they would otherwise touch,the effect is to produce an axially laminated magnet. Practically themagnet ends may be coated with epoxy or varnish during assembly orspacers used. An approximately equal length and width of each magnetwill be found to give a good reduction in losses while not undulycomplicating manufacture. This method is unlikely to work usefully inmotors of conventional length to diameter ratios as the magnet face areais already relatively small.

Other types of PMSM construction are possible, while maintaining thelarge gap. A slot-less construction in which the laminations become astack of rings, or are replaced with a magnetically permeable tube,requires much more magnet material and will normally be found uneconomicfor submersible pumping.

It is also possible to design the PMSM so that the slots are fullyclosed or almost fully closed in the vicinity of the lamination tips210. This ensures the retention of the winding without use of insulatingretaining wedges that may degrade. It also reduces the cogging torque,that is the alternating accelerating and retarding torque developed asthe magnets come into and out of maximum overlap with the teeth.

For the purpose of maximum power output at high efficiency it isnecessary to optimise the electromagnetic design. Unlike conventionalsubmersible pump induction motors, which invariably have two poles forthe reasons given above, it will be found that the optimum number ofpoles is usually six or more for PMSM motors up to approximately seveninches (17.5 cm) in diameter. Four poles will give an acceptable outputfor smaller motors but even more poles are preferred for larger motors.The higher pole count allows the flux density in the stator laminationsto be better distributed so that the amount of steel in the outer areas211, may be reduced. This permits the area of the slots 207, and hencethe amount of copper in the windings, to be increased. When the highfrequency restrictions discussed below on drive output are considered itwill be appreciated that, in larger motor sizes, higher pole counts aremore demanding of the drive. A limit may be reached where acceptingadditional stator lamination outer material is appropriate to make thedrive practical. Conversely as taught in U.S. Pat. No. 6,388,353, withdrives and step-up transformers typical of oilfield induction motortechnology, a high pole count motor permits operation at low speed andhigh torque for progressive cavity pumps. For example, a ten-pole motordriven at a frequency of 60 Hz will rotate at 720 rpm.

FIG. 5 shows a representative cross-section of a PMSM motor of thepresent invention constructed using known technology in the field ofsubmersible induction motors. A plurality of rotor assemblies is used toachieve the desired output power, the assemblies being rotationallylocked to a common shaft 201 running continuously through the electricalsection of the motor. Shaft stability is ensured by bearings 401 betweeneach rotor assembly. These bearings 401 are commonly made of twoconcentric rings running freely one over the other, one keyed axiallyand rotationally to the shaft 201 and the other locked to the statorbore using thermal expansion caused by the motor's self-heating, orpegged in some way.

U.S. Pat. No. 4,513,215 and U.S. Pat. No. 4,521,708 teach means added tothe bearing outer ring for pegging or gripping the bearing outer ring toprevent rotation during motor start up, before thermal expansion hastaken effect. However the larger shaft diameter made possible with PMSMmotors necessarily reduces the bearing outer ring wall thickness so thatsuch known methods cannot be used with such motors. FIG. 37 shows abearing outer ring 3704 suitable for use with PMSM motors utilizing aspring clip 3702 fitted to the stator which engages in a shallow axialgroove 3701 in the bearing outer ring 3704 (as may be best appreciatedby referring to the inset view showing a section taken normal to theplane of the drawing). The spring clip 3702 is preferably an opencircular spring, such as a commercial circlip or steel wire, since thisprovides a natural axial resilient lead-in as the spring clip 3702engages the bearing outer ring 3704. When the bearing outer ring 3704 isinserted into the stator bore, in all probability the groove 3701 willnot be opposite the spring clip 3702. The resilient lead-in allows thespring clip 3702 to push back to allow bearing insertion. When the motorstarts, the bearing outer ring 3704 will rotate until the groove 3701comes opposite the spring clip 3702, allowing it to expand and engagethe groove 3701, thereby preventing further rotation. A spring loadedpin or cantilever may also be used. In normal construction the stator ismade of brass or possibly steel laminations 403 at the bearing sections.To make the stator-mounted spring clip 3702 practical, these laminationsare preferably replaced by a single thick block 3703, cut as if it werea very thick lamination (slots not shown in FIG. 37). This may be acasting. The spring clip 3702 is then mounted in a pocket in the blockso that it cannot fall out during assembly. A small peg 3705 preventsthe spring clip 3702 rotating.

An improved method of assembling the rotor illustrated in FIGS. 7 and 8simplifies the bearing assembly and also the means of affixing the rotorsleeves 203 to the motor shaft 201, using a reduced number of parts.

The need to assemble the rotor with long solid sleeves 203 presents aproblem in that a stable fit to the shaft 201 is necessary, but therequired shaft straightness for closely fitting sleeves 203 to passsmoothly over the shaft 201 during assembly is very demanding. Thepreferred means of assembly is to use support rings 411 as shown in FIG.7. These rings 411 are a close fit on the shaft 201 but, being short inlength, will slide easily over it. Lands 415 provide a concentric fitfor the sleeves 203 and shoulders 416 provide an axial abutment. Thebore of the sleeve 203 is only a loose fit on the shaft 201. As shown inFIG. 8, one or more sleeves 203 and support rings 411 may be threadedonto the shaft 201, and axially constrained by convenient means such assnap rings 414. Each sleeve 203 must be rotationally fixed to the shaft201 in order to transfer the motor torque, and each ring 411 must beprevented from rotation on the shaft 201 in order to eliminate wear.Referring again to FIG. 7, a key 413 may be provided to accomplish this,in which case, during assembly, the ring 411 is first slid onto theshaft 201, then the key 413 is inserted into a groove 419 in the shaft201 and within a locating notch 418 in the ring 411. The sleeve 203 hasan internal groove 417 so that, when it is slid onto the shaft 201, itbecomes rotationally locked to the shaft 201 by the key 413 and alsoprevents the key 413 from subsequently falling out.

The use of a relatively short key, such as the key 413, ensures that thetorsional stress in the sleeve 203 is limited to that caused by thetorque generated by the magnets on the same sleeve 203. In a long motor,the portion of the shaft 201 under the sleeve 203 nearest the output endof the motor will carry a high torque accumulated from all the othersleeves. Particularly where multiple motors are connected in series toincrease power output, this torque can be very high. If the accumulatedstress in the shaft 201 were to be shared with the sleeve 203 by way ofa long key, there would be a risk that the magnets, being brittle, wouldfracture. (In submersible induction motors long keys are used tomaintain all the laminations of the rotor in alignment, as well as totransfer torque.)

A further consequence of very high torque is that twist in the shaft 201may cause sleeves 203 at opposite ends of the shaft 201 to come out ofalignment with each other and hence with the stator, with the resultthat the sleeves 203 cannot at the same time produce maximum torque.U.S. Pat. No. 6,388,353 suggests mounting the sleeves on the shaft withan angular skew relative to one another so that, when twisted in use,the sleeves are brought back into alignment. Alternative methods thatcan be used with series-connected motors are (i) to stagger the angle ofeach shaft to the next in line, such as by cutting the splines at theends of the shaft with a small angular offset relative to one another,or (ii) to connect the housings to one another with a small angularoffset. Within a single stator, the simple expedient of twisting thestator will effect a compensating correction to a twisted shaft, and issimilar to a well-known technique for reducing motor cogging torque. Thecompensation of all these methods has variable effectiveness as the skewis fixed at one angle to compensate the angle of twist at one level oftorque, and cannot therefore be correct at other levels of torque. Itshould be noted that the amounts of twist referred to are very small,typically less than a degree, and the problem may not be significant ifthe motor is designed to maximise shaft diameter and hence resistance totorsion. Accordingly it is preferred to design elongated motors so asnot to suffer from excessive shaft twist.

The sleeve 403 carries, or is integral with, the shaft bearing. A ring407 of bearing quality material, such as that marketed under thetradename Deva Metal, may be pressed onto the ring 411. The outer ring420 of the bearing runs on the ring 407, and is axially captured bythrust washers 421 which themselves are captured between the sleeves 203and the support rings 411. Alternative arrangements for the bearings, inwhich for example the support ring 411 is made entirely of bearingmaterial, eliminating the ring 412, are possible within the scope of theinvention. Similarly the outermost rings 411 may be of modified shape astheir outermost ends do not mate to rotor sleeves.

Substantial heat, of the order of 100 W, will be generated within thebearing. This heat is transferred to the laminations and thence themotor housing by two main means, namely conduction through the outerring 420, and conduction through the support ring 411 and the rotor. Inthe latter case heat passes through the magnets near each bearing andacross the oil-filled rotor/stator gap. This second path, though lessdirect than the first, will significantly raise the magnet temperature.A thermal insulator in this path between the bearing running surface andthe magnet, such as may be provided by making the support ring 411 ofceramic, will increase the thermal resistance in this path, and thusreduce the magnet temperature rise.

In a completed PMSM the rotor is centred by the bearings 401 and so ismagnetically balanced. Particularly when the motor is installedvertically the bearing loads will be very low, and the bearings 401,which necessarily run hydrodynamically for maximising lifespan, maybecome unstable, resulting in shaft whirl and other vibrations. In thepresent invention therefore, the bearings 401 must be designed to createsufficient internal pressure to remain stable and hydrodynamic at lowshaft load. FIG. 9 shows a means of achieving this in which a proportionof the length of each bearing 401 is etched or machined with spiralgrooving 409. The grooving 409 swirls the oil within the bearing 401 atinterface 407, increasing bearing pressure and enforcing stability. Thelength of grooving 409 is a means of varying the pressure. The grooving409, being inherently a miniature pump, controls and also promotes flowof oil through the bearing 401, assisting in cooling and cleaning it.Alternative bearings, such as known bearings with non-circular bores,may also be used to achieve stability.

A further means of purging oil through the bearings 401 is to bore themotor shaft 201 for the introduction of oil throughout the length of themotor to cross bores 406 at each bearing 401 as shown in FIG. 7.Utilising an impeller or cross drillings on the shaft 201, preferably byway of an oil filter thereupon, oil is forced into the shaft 201,through the bearings 401 and then returned by way of the rotor-statorclearance 409. Pilot bores 410, as shown in FIG. 7, or grooves in thebearing housings or the stator bore may be provided to assist thisreturn path, as will unfilled interstices between the magnets on therotor. The bearing running clearances, being small, resist and therebylimit the flow of oil from the shaft 201. This is particularly the casefor plain bearings. Because spiral groove bearings control their rate ofoil intake, it is preferable to arrange a copious supply of fresh oilnear the inlet end of the bearing 401, such as by making the cross bores406 near and at least partially beyond the inlet end of the spirals.Then there is no need to force the oil through the bearings 401. Insteadthe oil moves freely through the bores 406, circulating past thebearing, while the bearing ingests the portion of the oil that itrequires. Alternatively or additionally, oil that is flowing axiallythrough the rotor-stator annulus would not normally help to lubricatethe bearings 401 as no pressure is developed to force it into them. Thespiral groove bearings will benefit as they ingest from the flow. Thismethod thereby separates the general circulation of substantial rates offresh and cooling oil from the individual bearing lubrication process.It is generally applicable to any type of elongated fluid-filled motor.

If the stator bore is of constant, carefully controlled diameter, thenthe rotor assembly, complete with bearings, may be slid into the stator.The stator thereby provides the outer support for the bearing outerrings 420. However this known arrangement necessarily requires thebearing rings to rub the stator bore during insertion, with the possiblerisk of abrasive damage.

Alternatively the stator bore may be made of smaller diameter 403 in theaxial sections opposite the bearings, such that the reduced bore lieswithin the stator to rotor clearance, as indicated by the broken lines408 in FIG. 7. In the assembled position shown in FIG. 5, the bearings401 are shown in contact with these specially reduced sections 403 ofbore.

This means of assembly is not immediately suited to PMSMs due to theextremely high side magnet forces between the rotor and the statorcaused when the rotor becomes slightly eccentred in the stator bore.This is well known in small industrial motors where external fixturesare used to handle the forces involved during insertion. In an elongatedmotor the problem is very serious since it is not effective to supportthe shaft from each end. It may be found from electromagneticcalculations that a force of thousands of Newtons per metre of rotorlength and millimetre of deflection may be produced. A much smallerforce is sufficient to bend, or deflect, a shaft held only at its ends.Any deflection increases the side force leading to more deflection. Thusif the rotor bearings mate only to a restricted stator bore, then formost of the insertion process they will not provide support to therotor, and the rotor will deflect until the bearings touch the statorbore between the restricted sections. It would be difficult to slide therotor into the motor, and also the bearing outer diameters will now beoffset from the restricted bore section 403, so that the restrictionsbecome obstructions.

FIGS. 24 to 29 show representative means of the invention for overcomingthese problems. More particularly FIG. 27 is a cross-section of thestator bore in the vicinity of the bearing restricted bore section 403.The restriction surface 2101 is interrupted by three equiangularlyspaced cutaways 2103 that take the bore back to the normal diameter ofthe lamination tips 210. FIG. 29 is a cross-section of the bearing outerring incorporating three equiangularly spaced rebates 2105 andintermediate lands 2104 corresponding in position to the cutaways 2103of the restricted bore section 403, the rebated surface 2102 having thesame diameter as the restriction surface 2101, and these surfaces 2101,2102 mating when the rotor is finally installed and the bearings are inthe restricted bore section 403, as shown in FIG. 24. The outer surfaces2113 of the bearing outer ring are a sliding fit with the laminationtips 210. At interim positions during installation, as shown in FIG. 25,the outer surfaces 2113 provide a good degree of centralisation of theshaft 201 between each rotor assembly. This mechanical support ensuresthe rotor side forces remain acceptably low during rotor insertion, andmake it possible to insert the rotor assembly without damage. Theexternal fixture will necessarily constrain the axial movement of therotor to prevent it being pulled into the stator by the magnetic force.It will be apparent to a skilled person that, by suitable shaping of theleading edges of the rebates 2105, the alignment of the rotor bearingsto pass through the bore restrictions may be facilitated. However, in amotor with many bearings, this will remain troublesome. The largerrotor-stator gap in a PMSM permits a means of alignment to be used inwhich each bearing ring is bored so that a stout wire, such as atempered steel wire, may be threaded through each bearing. Keeping this,or a guide strip in a groove cut in the surface 2103, taut will greatlyfacilitate the alignment of all the rebated bearings as the rotor isinserted.

FIG. 4 shows a first step in the known, labour intensive method ofmanufacturing an electric submersible induction motor stator. The looselaminations 206 are threaded onto a mandrel 303 and inserted into thehousing, being prevented from escape by an internal ring 301. Theshoulder 304 is then used to compress the lamination stack, which isfastened in place by a second internal ring 302. The housing lengthextends beyond the laminations considerably further than theillustration shows, in order to leave space for the stator winding endturns and for mechanical components. Winding the stator is laborious asthe conductors have to be threaded axially through the slots back andforth, turned around, wrapped in additional end insulation andinspected, all taking place inside the housing ends. Furthermore, thelaminations necessarily are a relatively loose fit in the housing inorder to be able to slide them in. This leaves a significant thermalcontact resistance between the laminations and the housing, whichimpedes heat transfer, raising the motor internal temperature and hencereducing its reliability. During motor start up, the torque reaction onthe stator is transmitted to the housing by way of rings 301 and 302,and, when the motor has warmed up, the stator expands so that it gripsthe housing along its length. It is a significant mode of failure duringstart up for the middle section of the stator to twist between the endsportions, thereby damaging the windings, and the arrangements describedbelow are effective in preventing such failure.

The preferred embodiments of the present invention radically change themethod of assembly of elongated motors. Firstly the more conventionalapproach to making small non-submersible motors is adopted, in which thestator is wound before insertion into its housing.

A shrink fit of the finished stator is used to ensure high contactpressure with the housing, reducing thermal contact resistance andthereby minimising the internal temperature of the motor. However theinvention provides for the special circumstances of an elongated motorthat has internal bearing surfaces that may be on a reduced stator borediameter. FIGS. 26 and 28 illustrate a means of aligning the laminationsready for winding utilising a rebated mandrel 2107 having outer surfaces2105 that are a close but sliding fit in the stator bore. The rebates2106 are clear of the restriction surfaces 2101. This mandrel 2107provides a centring surface for all the laminations, including thoseused for the bearings. A simple nut and shoulder on the mandrel 2107 issufficient to clamp the laminations ready for winding by threading ofthe wire through the slots. The shoulders of the rebates 2106 may bepartly tapered to bring the bearing laminations into rotationalalignment. However, in order to reduce sliding friction when the mandrel2107 is eventually removed, the surfaces 2105 are preferably reduced tothin ribs or have ridges that reduce the contact area. If thelaminations are open to the stator bore, or if the tips have notches onthe internal bore, such features on the mandrel 2107 or inserts mountedthereupon may be used to rotationally align all the laminations.

The laminations may be welded together on their outer diameter so as tomaintain the close stacking of the laminations, or some other knownmeans may be used. After winding and possibly varnish impregnating, thestator may be ground on the outside diameter to make it a close fit inthe housing. Preferably the housing will be pre-expanded in order that,after insertion of the stator, the housing will relax to a shrink fit.The pressure of contact will then greatly reduce the contact thermalresistance. A means of expanding the housing for assembly is to pre-heatit. Conversely, to repair a stator, or to recover the housing forre-use, it will be necessary to expand the housing with the stator insitu. The length of the stator renders the required force to be too highfor a press tool to be used. A preferred means is to use an inductionheater, which essentially comprises an electrical coil that slides overthe housing, the coil being connected to a power generator ofappropriate frequency. It is known from the theory of induction heatingthat, by choosing a frequency such that the skin depth of radiation inthe housing does not penetrate through the housing, energy may berapidly and selectively imparted to it without penetrating the stator.This provides a time window in which the housing will release the statorand the stator may be extracted before it becomes heated by diffusionand expands to re-establish the lock. This method is suited even toelongated motors.

A further method with great advantages in terms of manufacturing andrepair cost, as well as in terms of reliably preventing the stator fromrotating within its housing, is to mechanically lock the stator to thehousing with an anti-rotation device or devices. One possibility is toprovide an axial key 3603 between the stator and the housing 202, asillustrated in FIG. 36. The housing 202 is formed with an axial groove3601, such as by milling or broaching, and the stator is formed with acorresponding axial groove 3602. The axial key 3603 or series of keysfits in both grooves 3601, 3602 so as to prevent the stator from turningrelative to the housing 202. This technique avoids the need to eitherpress fit the stator with great force or to shrink fit the housing overit, both of which are inevitably time consuming in manufacturing termsand require specialist equipment. With the keyed housing there is nopossibility for breaking free. It will be apparent that there are manyother possible arrangements that can be adopted using keying, such asleaving an integral raised feature on the circumference of thelamination which locates in the groove 3601. Where the laminations arepre-bonded into shorter lengths the anti-rotation device may be appliedon a per length basis.

The embodiment disclosed has the further advantage over conventionalconstruction of induction submersible motors that, by providing easyaccess to the end-windings, the highest quality winding procedure may befollowed and the results easily inspected. This technique is applicableto all types of elongated motor.

FIG. 30 shows an improved reliability and improved performance method ofwinding a PMSM. The winding is a known short-pitched concentric winding,which is not normally suited to industrial PMSMs or induction motorsbecause the back-emf waveform is far from sinusoidal. It may however bedriven by a variable speed drive in which the back-emf waveform is takeninto account, and it is particularly suited to the variable speed driveembodiments of the present invention that are disclosed below. In theexample shown the motor has three phases A, B, C. The laminations 2201have six slots for an eight-pole rotor. For phase A, coil 2202 is woundthrough adjacent slots so there is a single tooth 2210 separationbetween the coil sides. Identical coils 2203 and 2204 are wound onalternate teeth for the other phases B and C, where the hatchingemphasises the extent of the completed coils. The slots are shown closedat the stator bore as this is preferable.

One advantage for reliability is that there are only three coils, oneper phase. Consequently there is no crossing of phase windings at theend turns. The end turns 2205 possess further advantages. The end turns2205 fall naturally within the radial limits of the slots and are short.This minimises risks with insulation chafing, provides a short path backto the stator for conduction cooling, and has minimal energy waste inunproductive copper. This in direct contrast to double-layer lapwindings, as used in induction submersible motors. In these the endturns need to expand beyond the radial limit of the slots and/or becomevery long in order to accommodate the wire crossings between layers andphases. Since the winding area is constrained by the housing internaldiameter and the bearing/rotor outer diameter, the problem is severe.

A winding with six slots and eight poles as described above providesphase separation in the slots. However, at high speed, the high polecount makes heavy demands of the variable speed drive as describedbelow. Furthermore the self-inductance of each winding is high,requiring more drive voltage to overcome its reactance for a given motorcurrent, which is manifest as a poor power factor. A compromiseembodiment of six poles and nine slots for a motor having three phasesA, B and C, as shown in FIG. 31, is satisfactory and preferred. In thisembodiment three series-connected coils are provided for each phase, andeach slot accommodates the coils of two phases. However, since thesecoils are wound around separate teeth, they are naturally spaced apartby a gap 3106 as indicated in FIG. 31 and can be well insulated fromeach other.

A preferred improvement to the laminations where two coils are adjacentin the same slot is to introduce partial teeth 3110 shownrepresentatively in FIG. 31A. These teeth have little effect on themotor magnetic circuit as they do not form a closed loop around thecoils. However they form an intermediate path for heat transfer to theoutside of the motor, and, if the motor body is held near the potentialof the neutral point, there is less strain on the insulation betweeneach of the coils and the tooth 3110 than between the coils of differentphases.

A further, preferred feature, that may be used where the coils arepre-fabricated and accommodated in a common slot including partial teeth3110 for separating adjacent coils as shown in FIG. 31A, is theprovision of slots that are substantially shaped to conform to thecross-section of the coils in order to provide close thermal contact andmechanical support between the coils and the surrounding laminations.Such an arrangement is shown in FIG. 38 in which only one of the coilsis shown within the slot, and the coil comprises four coil sections 3801preformed from rectangular wire, each individual coil section beingencapsulated within a respective layer of insulation 3803 and the coilsections being fitted together to form a rectangular bar which is itselfencapsulated within an overall layer of insulation 3802.

A further, and preferred, aspect of the present invention is shown inFIG. 32, which, for convenience in drawing, is again shown based onsix-slot laminations. In this case the laminations are made in twoparts, as if split in the vicinity of the root of the slots so that amulti-tooth inner part 2207 and a circular outer part 2208 are obtained,as shown separately on the right hand side of the figure and fittedtogether on the left hand side of the figure. These parts 2207, 2208 arepreferably made from the same piece of material and are thus of closelysimilar thickness. In the final assembly this will reduce bridging ofthe tooth tips between laminations due to thickness mismatches. In theassembly process the lamination parts 2207 are assembled onto a mandrel,and separately wound and formed coils, preferably vacuum pressureimpregnated and over-wound with protective insulation, are then simplyslid over the lamination teeth 2209 of the parts 2207. Sometimes varnishis considered unsuitable in the face of hydrolysis from moisture ingressinto the motor. In these cases, wire insulation such aspolyetheretherketone may be used in loose coils, whilst still beingwrapped to resist chafing in the electromagnetic fields of the motor.Conveniently stacks of outer laminations 2208 bonded together or weldedalong their exterior are slid over the wound core to complete thestator. While it is possible to heat-shrink these onto the wound core,the housing shrink fit disclosed above will also apply the lightcompression necessary to ensure good mechanical stability of the stator.

The outer lamination stacks may instead be made as magneticallypermeable tubes of cast insulated iron powder, with the advantages ofoffering a smooth surface where the inner and outer parts of the statorcome together, and of economy of materials. Each tube may be made bycombining smaller arcuate segments, to reduce the size and cost of thecasting, such an arcuate construction being unfeasible with laminationswhich would effectively be small steel fragments. The partial teethdisclosed above may be incorporated into the outer ring of the splitlamination. The disclosed multi-part stator must also be multipart oropen-slotted outwards at the bearing sections 202 to permit loading ofthe winding from the outside.

This method of assembly translates the known advantages of form-woundcoils used in physically large industrial motors to the difficultelongated small diameter geometry of submersible motors.

Form-wound coils for lap wound large motors are manufactured separatelyfrom the laminations and are then inserted into rectangular slots opento the stator bore. In submersible motors there is insufficient workingspace in the bore of the laminations to load the formed-coils ready toinsert into the slots, and for high speed motors the open slots wouldcause substantial losses.

By opening the lamination slots from the outside, the invention permitsformed coils to be used while not incurring these problems. Theparticular advantage of the concentrated winding is that the formedcoils are very simple and that the large winding slots for the smallsize of motor facilitates the use of semi-rigid rectangular orwedge-shaped copper wire.

With formed coils the wire is bent once at any position as it is wrappedto form the coil, unlike the conventional process for winding elongatedmotors in which the wire is threaded back and forth through the slots.Consequently a much more rigid wire may be used, as there is no workhardening and insulation damage that would occur if it was attempted towind conventionally, with repeated bending. Rigidity solves the knownproblem in elongated motors of wires crossing within a coil deep insidethe stator. It provides a non-rubbing and stiff end turn assembly. Roundwire is known to give a very poor copper fill factor in a slot comparedto rectangular wire, essentially because the latter packs togetherbetter. Typically the thermal conductivity from the copper through itsinsulation back to the lamination is improved also. With a high copperfill the motor will have much reduced internal heating compared to aconventionally round wire wound motor. This is a source of improvedreliability, or alternatively of higher torque for the same temperaturerise.

While rectangular wire is preferred, it will be appreciated that formedcoils made from round wire will nevertheless be superior to round wireconventionally wound by threading through a stator. For a lower polecount motor, such as the commonly used two-pole induction motor, thewinding pitch is necessarily substantially half the circumference of themotor. This means that many end turn crossings are unavoidable.

The flexibility of round wires is beneficial in this case, whilstretaining the key advantage disclosed above of prefabricating the coils.

Formed coils that are fully encased in insulation over the portions thatenter the stator slots do not require insulating slot liners.Furthermore it is not necessary to impregnate the coils after insertionto complete the insulation and secure them into the slots, provided thatthe insulation is impregnated or encapsulated prior to insertion and theslots are shaped to retain the coils as disclosed above. The means thatthe present invention not only allows the stator to be removed from themotor housing, but permits the winding to be disassembled from thestator. Axial movement of the coils can be prevented by insertinginsulating blocks between the end turn loops and the stator.

Despite the many advantages of the split lamination construction, itdoes require careful design and attention to manufacturing to ensuresatisfactory engagement of the stator parts. All the aforementionedadvantages for windings may be obtained when the number of coil turns isnot too large, and especially for short-pitched coils, by using one-partlaminations as will now be described with reference to FIGS. 39 a and 39b. A four-turn coil is made first from four U-shaped coil sections 3901bound together by a layer of insulating material 3902 over at least thestraight parts of the coil sections 3901 as shown in FIG. 39 a, eachsuch coil section being termed a hairpin coil. During manufacture theopen ends of the U-shaped coil sections 3901 are inserted directly intothe laminations and bearing sections from one end without requiring themto be divided into two or more parts. Once this has been done the coilloops may be completed by joining appropriate ends together 3903 asshown in FIG. 39 b, for example by brazing directly or using bridgingpieces. These joints may then be covered with insulation, such asinsulating tape, and impregnated or encapsulated. It will be appreciatedthat the details of the hairpin structure may be varied in a number ofdifferent ways within the scope of the invention. It is not necessary inany of the coils for the start and finish of a coil to be at the sameend of the structure.

Referring again to FIG. 5, the disclosure of the described embodimentshas been based on a single wound stator. Commonly, for higher power,multiple housed motors are combined in series. FIG. 6 illustrates anembodiment in which two or more stator sections are accommodated withina single housing 202, in this case with one stator section per rotorsection. The corresponding phase windings 214 of each stator section areconnected in series while integrity of synchronicity of the sections isobtained by rotationally aligning the stator and rotor sections. Therotor sections are easily aligned on the shaft by keys.Disadvantageously, in a lap wound motor, the end turns will consume alarge proportion of the overall motor length and the reliability willdiminish in proportion to the extra end turns. Also, for a high speedmotor, the distance between bearings will become large and possiblynecessitate a larger number for shorter stages to maintain mechanicalstability of the rotor. However, in the preferred embodiments disclosedabove in which concentrated windings are used, the penalty foradditional end turns is much reduced and the motor is feasible fromreliability and performance points of view. In this case the practicaladvantages are the possibility of manufacturing a large variety of motorpowers from a basic stator length and the relative ease of windingshorter stators.

The stator sections may be carried and aligned on a common mandrel forinsertion in the motor housing 202, similarly to the foregoingdescriptions for a single stator. The stator bore restrictions in whichto house the bearings 401 are replaced by housings 404 concentric withthe individual stator sections. Concentricity is maintained by the motorhousing 202 when the entire assembly of bearing housings 404 and statorsections are inserted. The series connection 405 of the windings 214 ofthe stator sections is preferably achieved by permanent means such asbrazing. The use of connectors, while possible, reduces reliability. Itis a feature of the invention that winding before insertion permitsthese connections to be made and inspected beforehand.

High speed multi-pole PMSMs present a variable speed drive problem thatthe present invention addresses as described below. The origin of theproblem is that the base electrical frequency that the drive mustgenerate is the product of the number of motor pole pairs and the numberof shaft revolutions per second. A standard induction motor having twopoles and turning at 3600 rpm therefore has an electrical frequency of60 Hz. A PMSM in accordance with the invention rotating above 4500 rpmhas a much higher frequency. At 7200 rpm and six poles, the electricalfrequency is 360 Hz. This six-fold increase is a step change inoperating conditions for electric submersible pumping systems and wellbeyond the range of general industrial drives.

FIG. 10 shows a known electrical representation of a balanced PMSM withthree phases a, b, c and isolated neutral. Referring to phase a,reference numeral 701 is a motor terminal to which the voltage V_(a) isapplied. Current I_(a) indicated by the reference numeral 702 flows intothe motor winding which has resistance R indicated at 703 and aneffective inductance L indicated at 704. The effect of the permanentmagnets rotating past the stator winding is to induce an electromotiveforce (EMF) E_(a) indicated at 705. The other phases b and c may bedescribed in the same way with appropriate substitution of indices. Thethree phases are joined together at the neutral point N indicated by thereference numeral 706.

It will be appreciated that multiple motors, or stators, may beconnected electrically in series so that the resistances, inductancesand EMFs add to make a single equivalent larger motor with a commonshaft. Placing the terminals in parallel is also possible but posesdifficulties in controlling currents between all the windings. Morerealistic motor models in which for example the EMF source andinductance are lumped together as an element that calculates the timerate of change of the internal flux linkage, and in which magneticsaturation is taken into account, are all refinements which do notaffect the present invention. The number of phases, three, is wellsuited to the task of electric submersible pumping motors, but is notlimiting.

An idealised PMSM as described with reference to FIG. 10 producessinusoidal EMF, with each phase 120 degrees apart, and is driven by athree-phase sinusoidal voltage source. FIG. 11 shows graphically how asinusoidal voltage V, 802, applied to the motor with suitable amplitudeand in the presence of the motor EMF E, 801, will result in a phasecurrent I, 803. The source may also be current-controlled in which caseV is the consequence of I and E.

The sinusoidal nature of the electrical quantities is ideally suited tothe task of electric submersible pumping. This is because the smoothlyvarying waveforms do not cause damaging transients at the motorterminals, and because the motor torque can be shown to be constant withrotation, which reduces the likelihood of torsional vibration.

The construction of such a motor requires careful attention to thedistribution of the turns of the windings within the stator slots. Toproduce a sinusoidal EMF with a reasonable number of slots cut in thelaminations requires the turns from different phases to share slots andto be distributed among many slots. This immediately causes a reductionin reliability due to the potential for insulation failure in the manyend-turn crossings and due to the mixed phases. There is also the lossof useful copper due to increased insulation between the phases.

When the windings are made so that the phases are kept in separateslots, the back EMF will be more similar in form to E in FIG. 13. Thisis often referred to as a trapezoidal EMF. If the motor is driven withsinusoidal voltage or current the performance will not be as good as theideal sinusoidal PMSM made with the same amount of copper in thewindings.

FIG. 14 shows how a motor with trapezoidal EMF is driven, compared tothe sinusoidal motor waveforms of FIG. 11. The key feature is thatvoltage is applied to the motor across two phases only at a time whereasin a sinusoidally driven motor voltage is applied to three phases at atime. The two-phase driven trapezoidal wound permanent magnet motor iscommonly termed a brushless DC motor. The two phases are changedcyclically, as in AB, BC, CA, AB . . . . Whenever the phase pair ischanged, one phase is electrically disconnected. Since there will becurrent in the phase winding, the terminal voltage exhibits a voltageflyback spike 1002, known as a commutation spike. These spikes occurtwice per electrical cycle, on each phase. They present a seriouslimitation for the successful use of brushless DC motors in electricsubmersible pumping, since the voltage spikes lead to damagingelectrical conditions on long cables, as hereinbefore described. Theelectric submersible pumping system of the present invention drives allthree motor phases continuously such that damaging transients will notarise, without requiring the motor emf to be sinusoidal. It isconvenient nevertheless to explain the principles in terms of sinusoidalwaveforms, as the fundamental frequency component of the drive and motorelectrical quantities predominate in a detailed analysis.

FIG. 1A shows a block diagram of a drive circuit 111 comprising anadjustable voltage converter 113 and an inverter 114 for supplying drivecurrents at output terminals 901 at the surface for supplying the threephases A, B and C of the motor via the power cable extending down theborehole. The inverter 114 is supplied with an upper voltage at 904 anda lower voltage at 905, the difference between the upper and lowervoltages being commonly termed the link voltage.

FIG. 12 shows a schematic circuit diagram for the inverter 114 which iswell known. For each phase output there is an upper switch and a lowerswitch, representatively shown for terminal AA at 906 as 902 and 903respectively. By alternately turning on these switches, upper switch 902on and lower switch 903 off or vice versa, this terminal may be sensiblyconnected to either the upper voltage 904 or the lower voltage 905. Thisarrangement is termed a two-level inverter. It will be appreciated byone versed in the art of inverter design that multi-level inverters maybe made in which the terminals may be switched to voltage levelsintermediate between the upper voltage and the lower voltage, suchmulti-level inverters being usable in alternative embodiments inaccordance with the present invention. A filter is connected between theswitch terminals AA, BB and CC and the drive terminals A, B, C at 901,representatively comprising inductors 907 and capacitors 908. Thepurpose of the filter is to smooth out the rapid switching transitions,and thereby present a smooth voltage to terminals A, B, C. It will beappreciated that other filters, for example for the removal of radiointerference, may be added.

By contrast, in a brushless DC motor inverter, the filter is not presentand the motor is connected directly to the terminals AA, BB and CC. Onlytwo phases are active, that is only one switch is turned on, at a time,whilst the switches for the third phase both remain turned off asdescribed above.

In driving of the motor in accordance with the present invention allthree phase outputs are active at all times. In a sinusoidal variablespeed drive it is necessary to use pulse-width modulation (PWM) or otherswitching modulation scheme known in the art, e.g. hysteretic, spacevector, switching table, to create the effect of a sinusoidal outputcurrent. In the following description PWM drive is referred to by way ofnon-limiting illustration.

FIG. 15 shows one phase of the output of a PWM drive according to whichthe upper and lower switches of a phase leg are alternated with avariable mark-space ratio. The voltage curve shows the switching atterminal AA, whereas the superimposed phase current curve is seen besinusoidal with only a little ripple. Fourier analysis of the voltagewould show it to have a predominant fundamental component at the phasefrequency. Filter 907, 908 filters the voltage output of the drivecircuit so that only the fundamental smooth voltage is passed to thepower cable and thence to the downhole motor. This is therefore asuitable transient-free approach, in principle, for the PMSM submersiblepump application

However, to produce a high-power high-speed variable speed drive withsinusoidal output presents severe difficulties, as will now bedescribed. The method is best suited to trapezoidal or similar EMF butis also applicable to sinusoidal driving, the difference being in theharmonic content of the waveforms and hence the best use of availablepower capacity.

The majority of variable speed drive circuits operate at typical utilitysupply voltages of 380 V AC-690 V AC, since the power semiconductorsthat they use for switches are well proven and efficient. However, justas in utility power transmission, for efficient motor operation usinglong power cables, it is necessary to use Medium Voltages, commonly inthe range 1000 V AC-4000 V AC. Such voltages reduce the motor currentand hence the ohmic losses in the cable. The majority of variable speeddrive circuits for use with submersible pumps are therefore installedwith a step-up transformer on the output. These transformers are asource of additional power loss, direct cost, and are often large andoil-filled, requiring special environmental precautions and substantialspace. A wide speed range requires expensive core material for highspeed but also a very large core to prevent magnetic saturation ifoperation at low speed is also required. They are in addition to inputtransformers, commonly required as described below to reduce harmonicdistortion of the supply and to match to the available supply voltage.

A Medium Voltage drive circuit operates from a supply voltage directlyat the voltage which is required for the motors. It therefore eliminatesthe undesirable output step-up transformer but has certain limitationsfor the purpose of high speed pumping.

Medium Voltage power semiconductors when used for the switches 902 and903 of the drive circuit of FIG. 12 have large switching losses, i.e.unlike ideal switches they carry both current and voltage during thetime it takes to open or close the path to current. The losses areinherently proportional to the number of switching operations persecond. As an example, to turn on a switch at 3000 VDC assuming acurrent of 200 A might cause a loss of 1 J (Joule). If repeated 1000times per second, the heat created would be 1000 W. It is easy to seethat, once accumulated across all the switches of the drive, there wouldbe a substantial cooling problem and loss of efficiency.

To produce the quality sinusoidal waveform in FIG. 15, thirty switchingcycles per fundamental motor frequency cycle were used. FIG. 16 showsthe effect of reducing this to ten switching cycles per fundamentalmotor frequency cycle. The waveform is already of poor quality anddifficult to filter.

With high speed multi-pole motors the switching speed becomes too highto be economic with Medium Voltage semiconductors. For example, a highspeed motor with six poles operating at 7200 rpm has a fundamentalfrequency of 360 Hz, so that the drive should operate with a switchingfrequency of at least 3600 Hz just to achieve the quality of theresponse of FIG. 16, and preferably at least twice that. The normalrange for Medium Voltage power semiconductors is 500-1000 Hz. This iswhy Medium Voltage drives for two-pole induction motors, which need afundamental of 60 Hz at 3450 rpm, are typically specified at an upperfundamental of less than 90 Hz, far short of that needed for the highspeed motors referred to above. If a lower voltage drive is consideredas an alternative, then despite more efficient semiconductors, it toowill reach a switching limit at high power. Moreover the step-uptransformer has to be a more costly design as mentioned above.

The present invention overcomes these problems by deliberatelyover-modulating (non-linearly modulating) the output stage of FIG. 12 inconjunction with a variable voltage source such as shown in FIG. 19.Normally the PWM waveform in FIG. 15 may be used to produce a goodsinusoidal waveform until the peak value exceeds 4/π times the internaldrive voltage. If the depth of modulation is increased beyond this thePWM output will become distorted, that is the modulation will becomenon-linear, as shown in FIG. 17. This non-linearity is characteristic ofany modulation scheme used for driving a motor in accordance with thepresent invention where the output voltage is unable to follow the peakof the sine wave or other waveshape that is demanded, in simpleproportion. A particular case of over-modulation is to consider the peakvoltage as fixed to the upper and lower levels of the internal drivevoltage for substantial parts of each cycle, with pulse width or othermodulation used to progressively vary the output between the upper andlower levels of the internal drive voltage for the remaining parts ofthe cycle. It is also possible, within the scope of the invention, togenerate the waveform within the linear range of modulation,particularly at lower power levels.

The distorted switching waveform shown in FIG. 17 has several features.There are far fewer switching cycles than at lower modulation, and theseare at the lowest-current intervals of the fundamental cycle. Switchinglosses will therefore be much reduced even if the switching frequency iskept high to facilitate filtering. The filtered output voltage,obtained, for example, by filtering the single phase in isolation, isquite similar to trapezoidal, and is transient free as required. Whenthe filter is as shown in FIG. 12, it becomes a three-phase filter andthe phase-phase voltage applied to the motor will be found to be evenmore smooth. The output is usable for sinusoidal motors, with someunwanted harmonics, and is well adapted to the non-sinusoidal windingsof the preferred motors of the present invention.

FIG. 21 shows the benefits of over-modulation more clearly. Horizontalaxis 1702 is modulation depth normalised to 4/π, and vertical axis 1701is the heat loss of a typical switch. Curve 1703 shows the switchconduction loss, which is typically low for a submersible motor runningat 100 A. Curve 1704 shows the switching loss. It is high for normalmodulation levels, but reduces rapidly by a factor of three asover-modulation is increased. This represents a dramatic improvement andmakes Medium Voltage drives of the present invention suited to highspeed motors.

Since the amplitude of the drive voltage is fixed once over-modulationis employed, the only way of varying the motor voltage and hence thespeed is to vary the internal drive voltage V_(link) applied between theterminals 904 and 905 in FIG. 12 by means of the adjustable voltageconverter 113. There are many known circuits to do this, includingphase-controlled rectifiers and choppers.

However, the present invention seeks to make use of the specialcharacteristics of the pumps it is powering, in order to further improvedrive performance. In accordance with the power characteristic ofcentrifugal pumps as mentioned earlier and as depicted in FIG. 18, thepower output at half speed is only 12.5% or so of the full speed power,and therefore of little interest in the well for which the motor andpump are specified. Similarly, though less dramatic, in a positivedisplacement pumping system the power will be proportional to speed andmore than half power is normally required.

Therefore a properly specified drive can be assumed to be run most ofthe time above half speed.

A first embodiment of adjustable voltage converter 113 particular to thepresent invention incorporates a specially adapted variable voltagechopper source shown in FIG. 19 to provides an efficient means ofregulating the internal drive voltage, and hence the motor speed, overthe power range of interest.

In this circuit a first fixed supply voltage source 1401 is connected inseries with a second fixed supply voltage source 1402, and a chopper,comprising a switch 1403, a diode 1404, an inductor 1405 and a capacitor1406, is connected across the source 1402. By varying the duty cycle ofthe switch 1403, the voltage across the capacitor 1406 may be variedbetween zero and the fixed voltage of the source 1402. Since the voltageacross the capacitor 1406 is in series with the fixed voltage of thesource 1401, the voltage across the output terminals 904 and 905 may bevaried from the fixed voltage of the source 1401 to the sum of thevoltages of the sources 1401 and 1402.

When the motor is operating at low speed, as when starting and stopping,the power level and motor frequency will be low. Consequentlyconventional pulse width modulation by the drive output may be used withlittle penalty, with the chopper turned off, leaving the drive voltagefixed at the level of 1401. At full power output the chopper may be leftpermanently on. Therefore it has no switching losses in either case.

The switching losses of a chopper are proportional to switchingfrequency, input voltage and output current. The advantages of thearrangement shown are that the input voltage is only half that of thefill supply, and that the frequency of chopping may be set independentlyof the motor speed since it is used to produce the link voltage and notthe modulated drive output to the motor. For example, with a pump loadand high speed corresponding to high power, the chopper might beoperated at 500 Hz to limit switch losses, whereas the output stage inFIG. 12 may be switching, except when saturated, at 3600 Hz or more toproduce a fundamental frequency of 360 Hz. If the conventional modulatedPWM approach with a fixed internal supply were used, the output mighthave to be limited to 500 Hz, that is there would not even be one pulseper half cycle, resulting in an ineffective drive.

If a chopper were used across the full available supply voltage, thelosses would be doubled as the switching voltage would be doubled whilethe switched current remained the same. This may be acceptable for lowerpower low cost drives where the dual fixed voltage supply 1401, 1402 isnot implemented.

A further feature to optimise the drive, based on the characteristics ofthe electric submersible pump is to vary the chopper frequency. Inductor1405 is heavy, costly and has losses proportional to ripple current andaverage output current. As such it is an undesirable addition bycomparison with conventional drives.

The inductance value is usually chosen to limit the chopper ripplecurrent to a reasonable level. The ripple current is at a maximum whenthe chopper output is half its input voltage (50% duty cycle). At thesame time, because of the nature of the pump load, the output power willbe significantly reduced. Therefore at this condition the chopperfrequency can be relatively high, permitting a lower inductance value.As the voltage increases the output power will increase, and the chopperfrequency must be reduced to limit the switching losses. Since theripple reduces as the output voltage increases (higher duty cycle) butincreases as the frequency is reduced, it can be seen that a compromiseprofile of frequency versus power output can be found which allows amuch smaller value of inductance than would otherwise be the case,reducing the adverse factors of weight, cost and power loss. It is quitereasonable to reduce the value by a factor of two, or four if thechopper is connected across the full supply and not a portion of it.Thus variation of the internal frequency of the adjustable voltageconverter with output serves to improve efficiency and/or reduce thesize of components.

FIG. 20 shows a suitable circuit for the fixed voltage sources 1401 and1402. In this circuit the utility supply is first transformed by atransformer 1501 having two secondary windings 1502, the output of eachof which is fed to a three-phase rectifier and smoothing capacitor. Theresulting DC supplies are connected in series. By altering the relativeturns ratios so as to change the relative sizes of the voltages of thesources 1401 and 1402, the variable speed range of the supply may beadapted to particular requirements.

A particularly beneficial choice is when one secondary winding iswye-connected and the other secondary winding is delta-connected, andthe turns ratios are adjusted such that the rectified outputs are equal.In this case the current pulses taken from the supply from one capacitorare displaced in time with respect to the current pulses taken from theother capacitor. This known arrangement is beneficial to the supply asthe current pulses taken from the supply by the assembly occur twelvetimes per supply cycle and not six as when using a single rectifier.This substantially reduces the harmonic distortion imposed by the drivecircuit on the utility supply.

A further embodiment of adjustable voltage converter 113 in accordancewith the present invention incorporates a three-phase boost converter asshown in FIG. 35. This arrangement uses a two-level inverter as in FIG.9, but operated so as to absorb power from the input terminals 3501, U,V, W which in this case are connected to the utility supply or agenerator, and produce a voltage output between the terminals 905 and904. This circuit arrangement and variations thereof can use a greateror lesser number of switches. Their use in industrial drives is mainlyfor four purposes, namely (a) to draw a sinusoidal current from theutility supply which improves the harmonic content, (b) to adjust theutility supply power factor, (c) to permit the regeneration of powerfrom an over-running motor load to be returned to the utility supply,and (d) to provide a constant link voltage just above the maximum thatwould normally be obtained from an ordinary rectifier. The latter isalso the minimum link voltage that can be used for the circuit tofunction with these purposes. In this embodiment the three-phase boostconverter is used to produce a variable voltage between the minimumvoltage level and a maximum voltage level that is above twice thislevel, and therefore provides a suitable adjustable range covering themain power levels of interest. A particular benefit for smallersubmersible motors is that ordinary 380-480 VAC utility supplies can beboosted into the low Medium Voltage range, and therefore the MediumVoltage drives of this invention may be made without the cost of inputtransformers.

Other arrangements for efficient variable voltage are possible. If thefixed voltage sources 1401 and 1402 are kept separate and a chopper isplaced across each, and the chopper outputs are connected in series, onechopper may be kept fully on or fully off which the other chopper variesits output. In this way the entire voltage range may be covered withonly one chopper operating at less than the sum of the available supplyvoltage. It is also possible to connect the chopper across the fixedvoltage source 1401 and to connect the fixed voltage source 1402separately to the chopper output so as to add to it. Where theinterference to the utility could be tolerated, one or both of therectifiers in FIG. 20 could be rendered controllable by means of athyristor bridge. Three-phase chopper circuit arrangements are known andcan also be used.

It is desirable to dynamically control the drive, cable and motorarrangements to realise optimum efficiency. Unlike an induction motor aPMSM must be driven synchronously with its shaft rotation. With abrushless DC motor, for which two phases are driven at a time ashereinbefore described, there are numerous schemes to determine theshaft rotation without the use of rotation sensors (such as “SensorlessVector and Direct Torque Controls”, 1998, P Vas, Oxford UniversityPress) based on observing the effect of the voltage on the undrivenphase. With a PMSM, because all three phases are continuously driven,the shaft rotation must be determined in another way. In a factory, ameans is required to directly measure the instantaneous angular positionof the rotor using a shaft sensor, such as a resolver or encoder, and touse the result to control the phase of the voltage or current output ofthe variable speed drive circuit. However, apart from the uncertainreliability of such transducers, the additional cabling or other meansneeded to transmit the position information from deep in the borehole tothe variable speed drive circuit at the surface makes a sensorlesstechnique almost certain to be required.

It is possible to control AC PMSM motors without sensors utilising acomputerised or discrete component model of the motor, based on anelectrical equivalent circuit as in FIG. 10 or a physics-baseddescription, incorporating intimate knowledge of the motor'selectromagnetic design. The model is kept supplied with terminalvoltage, current and frequency information, which allows it to estimatethe motor's internal variables such as rotor position. In turn theseallow the control algorithm to decide how to adjust the drive output.These methods depend on an accurate model. Substantial effort is devotedto measuring the model parameters for a given motor before use, or byvarious monitoring means during operation. In the case of submersiblemotors, or in other applications where long cables are required, thecable resistance and reactance parameters must be incorporated into themodel. Furthermore cable and motor parameters are subject to change withoperating temperature and age. In the present invention it is shown howqualitative knowledge of the motor load characteristics may beintroduced so as to refine the rotor position estimate for a PMSMwithout having to measure these uncertain system parameters. A generalpurpose drive is designed to cover a wide range of loads and dynamicallyvarying conditions, as in a servo, and cannot assume particularproperties of the load.

The characteristic of the load that is required for the feature of theinvention now to be disclosed is that its power be steady for a steadyspeed. By averaging over a sufficiently long period random or short termload fluctuations can be accommodated. A submersible centrifugal pump,or a turbine, meets this condition.

Therefore, if the PMSM motor control is changed while keeping thefrequency, and hence synchronous speed, constant, the load power willremain unchanged. The optimum control condition will be when the driveoutput power, which is measurable, is minimised. For example if a pumpis tuning at a fixed speed and takes 300 kW from the drive at onecontrol condition, and 298 kW at another, then the second condition ismore efficient as there is less power supplied, regardless of what themotor and cable parameters are thought to be and regardless of what theactual pump power is, since it has not changed.

Suitable means of effecting these control changes to find the mostefficient system operating point are now disclosed. At constant speedthe internal mechanical friction losses will be fixed, so that thedominant variable loss that needs to be reduced to a minimum by thecontrol is the ohmic loss in the windings.

FIG. 22 shows the phasor diagram for the PMSM schematic shown in FIG. 10and corresponding to the waveforms shown in FIG. 11. This diagram, whichis known to those skilled in the art, refers to one phase of an idealbalanced motor with isolated neutral. The motor EMF amplitude E, denotedby the reference numeral 1801, is taken as reference. The phase currentI lags behind this by an angle φ. The voltage drop due to the windingresistance 1802 and the voltage across the internal inductance 1804 sumas vectors to equal the driving voltage V denoted by the referencenumeral 1805. The motor power output, ignoring internal mechanical andiron losses, is given by:P=3/2EI cos(φ)

The EMF E depends to a good approximation only on the motor speed, sothat, for fixed speed operation and since the output power is fixed bythe load, the quantity I cos(φ) will be constant. The broken line 1806shows the locus of constant output power. It is evident that minimumcurrent, and hence least loss in the resistance of the copper winding,≈I²R, occurs when φ is zero.

In open-loop operation of the PMSM motor a given three-phase voltage orcurrent is applied at a given frequency. The motor operates inaccordance with the phasor diagram at an angle φ, satisfying therelationships between the parameters. Operation is at some non-zero φ,and as a result the motor is never optimally efficient. If φ increasesbeyond approximately ±45 degrees, the motor operation becomes unreliablesince fluctuations in the load may increase the angle to the point wherethere is insufficient current available from the drive circuit tomaintain the output power. At this point the motor will lose synchronismand stall. If the conditions are as shown in FIG. 22, then increasingthe motor voltage will force φ to reduce and hence reduce the currentand ohmic losses. It will eventually reach zero, the most efficientpoint, and then become negative, when the current and ohmic losses willincrease again.

FIG. 23 shows this in terms of the input power 1902 of a representativemotor plotted against terminal voltage 1901, normalised to the mostefficient point of operation. The curve 1903 represents the average(real) input power and is the sum of the fixed motor output power andthe copper losses. The curve 1904 represents the volt-ampere power,which includes a power factor. It can be seen that, by varying the inputvoltage, a point may be found which minimises whichever power quantityis desired. Increasing the normalised input voltage from below unitychanges φ from positive through zero to negative.

Therefore the optimum operating point of the system at a given speed maybe found by varying the input voltage independently of particularknowledge of the motor or cable parameters or actual load power. Sincethe motor output power demanded by the pump for constant pump speed andfluid type is constant, the control observable is the input powermeasurable at the surface. Parameters corresponding to the age or natureof the motor and/or cable are not required.

This method has broader applicability. For example, if acurrent-controlled drive is used, then current may be used as thecontrol variable and the drive voltage and output power will vary as aconsequence. Alternatively, for a fixed amplitude of voltage or current,the phase of this quantity relative to the estimated rotor angle couldbe slightly changed. In this case the optimum condition will be when thespeed is maximised, since, from the characteristics of the load,increased speed corresponds to increased output power, and the maximumoutput power always occurs when φ reaches zero. (In FIG. 22, changingspeed changes the reactance X and the emf E in proportion.)

Practically the above method is applicable to PMSMs without thesimplified assumptions of FIG. 10, since the goal is simply least drivepower for a fixed output power. It is preferably implemented as a longtime average correction to an established closed-loop control, or as aslow adjustment for open-loop control, ensuring that load fluctuationsdo not cause false corrections. It will be evident to those skilled inthe art of control theory that it is possible to merge the input powerminimisation into an inner control loop, by weighting its importance toother error terms or using it as a constraint. In this way the controlloop keeps primary control over the range of φ that is permitted, whileaccepting a safe level of correction from the power-optimised control ofthe present invention. It should also be noted that, for realnon-sinusoidal motors and drives, the issue of torque ripple can beimportant, and a lesser or greater amount of correction might beempirically added to keep this to an acceptable level. Such anarrangement is applicable to any synchronous motor with suitable loadand thereby includes brushless DC motors and drives.

The improved winding and construction methods described above make itpossible to further extend the reliability of the entire pumping systemby means of cooperative duplication of failure prone electricalelements. The cost of replacing a failed system, and the loss of fluidproduction until the repair is complete, will far exceed the cost of theduplicate parts to be described.

FIG. 33 shows a variant installation according to which a singlesubmerged motor 108 is connected to two electric power cables 110 and110′, each connected at the surface to a corresponding motor drive 111or 111′. FIG. 34 diagrammatically shows the construction of the motor inthe installation of FIG. 33 in which two sets of motor windings areprovided, namely a first winding set 3001 connected to the cable 110 anda second winding set 3001′ connected to the cable 110′. The motor iswound as a six-phase motor, divided into two sets 3001 and 3001′ ofthree phases, each with its own neutral connection. The motor may eitherbe driven as a six-phase motor to its full power capability, oralternatively as a three-phase motor using either set of windings atreduced power.

The advantage of such an arrangement is that failure of the cable 110 orits splices or connections, or of the drive circuit 111 or of any of thecoils of the winding set 3001 will not affect any of the correspondingparts associated with the winding set 3001′. By using concentratedwindings the windings of the phase sets may more easily be kept wellinsulated from one another than with lap windings. In the simplest caseone coil per phase is wound over alternate teeth in twelve slots, thecorresponding phases of each set being adjacent to one another. In thisway the six motor leads exit the motor directly from their coils withoutcrossing.

The above arrangement of six phases split into two sets of three phases,though having practical advantages, is not limiting. However two-phasemotors still require three conductors in a cable, whereas motors with alarger number of phases require further cable conductors, which isundesirable.

A six-phase drive output circuit may be constructed by adding threeextra pairs of switches to FIG. 9. The phases operate at a 60-degreeseparation, rather than 120 degrees. However, such a drive sold as aunit is complex and is substantially only useful for a fault tolerantapplication. The preferred embodiment uses two adapted three-phase drivecircuits 111 and 111′, with the adaptation being made by means ofsuitable signal and power connections 3002 between the drive circuits.The signal connections must ensure that the corresponding output phasesof the drive circuits are 60 degrees apart. One drive circuit makes therotor angle calculation to produce the master phase signal used by bothdrive circuits. To ensure smooth running across all of the phases, theamplitude of the drive output voltage must be the same on each phase. Inthe case of the high-speed drives disclosed in the present description,the terminals 904 and 905 of the drive circuits may be connectedtogether so that their voltages are the same. One of many known powersupply sharing methods may be used to equalise the power supplied byeach chopper circuit.

In an alternative embodiment of the fault-tolerant pumping system, twomotors are connected mechanically in series on a common shaft, butpowered by separate cables and drives as before. The two motors may beoperated individually or simultaneously. In the latter case the drivecircuits must again be arranged to cooperate. This method is notapplicable in the majority of wells as usually the motor diameter is thelargest that can be fitted within the casing 103, and the motor cable110 is fed past the pump and into the top of the motor 108 immediatelybelow it. In such cases it is not possible to pass a second cable pastthis first motor to a further motor arranged at a deeper level.

The foregoing has assumed two duplicate motor sections that areidentical, as that is the simplest fault tolerant arrangement. However,with suitable changes to the control and drive levels, a plurality ofmotor sections cables and drives of different characteristics may beused within the scope of the invention, so long as they are controlledto the same shaft speed.

The electric submersible pump system of the present invention has broadapplication, particularly in the field of downhole wellbore operations.Drilling for wellbore fluid at large depths is typically restricted torelatively narrow boreholes, so the facility of the present invention toprovide the same motor power in a smaller overall package is immediatelyadvantageous.

A further application of the present invention is to compress wellborefluid in situ. It may sometimes not be required to immediately transportthe wellbore fluid to the surface from its underground reservoir, but tocompress it either for later recovery or merely to facilitate furtherexploration. Alternatively it may be required to transport the wellborefluid from a first subterranean location to a second subterraneanlocation, for the above reasons amongst others.

A recent development in mining operations is the application ofmulti-lateral wellbore systems in which a number of small diameterwellbores are drilled substantially horizontally from a centralsubterranean sump. Currently known pumping systems have significantdifficulties in pumping from lateral wellbores, whereas the pump of thepresent invention can still maintain a high output in such environments.In this case, wellbore fluid is transported from the multiple lateralwellbores to the central sump, where it may be recovered to the surfaceor compressed as described above.

As hereinbefore described, an objective of the present invention is toprovide a high-speed electric submersible pump, capable of operating atspeeds above the current maximum of approximately 4,000 rpm. Thestandard operating speed of embodiments of the invention intended forthe above applications is above 4,500 rpm, and an optimal speed,providing a marked improvement over current systems, is approximately7,200 rpm.

The present invention discloses a permanent magnet synchronous motorsubmersible pumping system. It will be appreciated by the person skilledin the art that various modifications may be made to the aboveembodiments without departing from the scope of the invention.

Reference should also be made to “The Technology of Artificial LiftMethods”, Vol. 2b, K. E. Brown, Penwell Publishing 1980, the contents ofwhich are incorporated herein by reference.

1. A method of pumping wellbore liquid, comprising: installing anelectric submersible pump, driven by an AC synchronous permanent magnetmotor, in a wellbore; and operating the pump at more than 4,500 rpm topump the wellbore liquid, wherein: the motor has at least three phases,and each of the at least three phases are continuously driven by aninverter that switches a voltage in the range of 1000 V-4000 V.
 2. Amethod according to claim 1, wherein the pump is a centrifugal pump. 3.A method according to claim 2, wherein the pump draws the wellboreliquid from a plurality of lateral wellbores into a central pump.
 4. Amethod according to claim 1, further comprising recovering the wellboreliquid to the surface.
 5. A method according to claim 1, furthercomprising transporting the wellbore liquid from a first subterraneanlocation to a second subterranean location.
 6. A method according toclaim 1, wherein the pump is operated at more than 5,000 rpm.
 7. Amethod according to claim 1, wherein the pump is operated at 7,000 to7,500 rpm.
 8. A method according to claim 1, wherein the pump isoperated at more than 6,000 rpm.
 9. A method according to claim 1,wherein the pump is operated at approximately 7,200 rpm.
 10. A methodaccording to claim 1, wherein: a power supply of the motor is located atthe surface, and the power supply models operation of the motor andcalculates a rotor position of the motor.
 11. A method according toclaim 1, wherein a power supply of the motor comprises a variablevoltage chopper.
 12. A method of pumping wellbore liquid according toclaim 1, wherein the motor is sealed from the wellbore liquid.
 13. Amethod of pumping wellbore liquid, comprising: installing an electricsubmersible pump, driven by a permanent magnet motor, in a wellbore; andoperating the pump at more than 4,500 rpm to pump the wellbore liquid,wherein the motor comprises: a rotor, comprising: a central shaft; aplurality of tubular elements spaced axially along the shaft; and aplurality of support rings spaced axially along the shaft between thetubular elements and serving to support the tubular elements on theshaft so that the tubular elements are rotatable with the shaft; aplurality of permanent magnets spaced about a circumferential outersurface of each of the tubular elements; a respective retention sleevetightly fitted over the permanent magnets on each tubular element so asto retain the permanent magnets on the tubular element; and a statorcoaxial with the rotor, comprising: a stack of laminations; and radiallyspaced coils wound around the stack, wherein the motor has at leastthree phases and at least two of the phases are driven at a time.
 14. Amethod of pumping wellbore liquid, comprising: installing an electricsubmersible pump, driven by a permanent magnet motor, in a wellbore; andoperating the pump at more than 4,500 rpm to pump the wellbore liquid,wherein the motor comprises: a rotor, comprising: a central shaft; and acarrier sleeve loosely fitted on the shaft; and rings closely engagingthe shaft and supporting the carrier sleeve; and a stator coaxial withthe rotor, comprising: a stack of laminations; and radially spaced coilswound around the stack, wherein the motor has at least three phases andat least two of the phases are driven at a time.
 15. A method of pumpingwellbore liquid, comprising: installing an electric submersible pump,driven by a permanent magnet motor, in a wellbore; and operating thepump at more than 4,500 rpm to pump the wellbore liquid, wherein themotor comprises: a rotor, comprising: a central shaft; and a pluralityof permanent magnets having axial ends; a carrier sleeve mounted on theshaft and bearing the magnets; a retention sleeve extending over themagnets; and at least one stress-relieving radially outwardly extendingabutment part on the carrier sleeve abutting an adjacent axial end ofthe magnets, wherein: the retention sleeve has at least one end portionturned in over the abutment part to retain the magnets in position onthe carrier sleeve without damaging the axial end of the magnet, and themotor has at least three phases and at least two of the phases aredriven at a time.
 16. A method of pumping wellbore liquid, comprising:installing an electric submersible pump, driven by a permanent magnetmotor, in a wellbore; and operating the pump at more than 4,500 rpm topump the wellbore liquid, wherein the motor comprises: a rotor,comprising: an elongate central shaft; and elongate permanent magnetsextending along the shaft, the magnets comprising axially laminatedparts to reduce eddy current losses; and a stator coaxial with therotor, wherein the motor has at least three phases and at least two ofthe phases are driven at a time.
 17. A method of pumping wellboreliquid, comprising: installing an electric submersible pump, driven by apermanent magnet motor, in a wellbore; and operating the pump at morethan 4,500 rpm to pump the wellbore liquid, wherein the motor comprises:a rotor; a stator coaxial with the rotor; a bearing mounting the rotorto the stator; a resiliently biased projection disposed on one of thestator and the bearing; and a receiver disposed on the other of thestator and the bearing, wherein: the projection is operable, by rotationof the rotor, to engage the receiver, thereby rotationally coupling thebearing and the stator, and the motor has at least three phases and atleast two of the phases are driven at a time.
 18. A method of pumpingwellbore liquid, comprising: installing an electric submersible pump,driven by a permanent magnet motor, in a wellbore; and operating thepump at more than 4,500 rpm to pump the wellbore liquid, wherein themotor comprises: a rotor; a housing; a stator coaxial with the rotor andmounted in the housing; an axial groove formed in one of the stator andthe housing; and an axial key engaging the axial groove, therebyrotationally coupling the housing and the stator, wherein the motor hasat least three phases and at least two of the phases are driven at atime.
 19. A method of pumping wellbore liquid, comprising: installing anelectric submersible pump, driven by an AC synchronous permanent magnetmotor, in a wellbore; and operating the pump at more than 4,500 rpm topump the wellbore liquid, wherein; the motor is driven from a powersupply through a variable voltage chopper and an inverter, and thevariable voltage chopper is provided with an input voltage comprising aportion of a total fixed voltage provided from the power supply.
 20. Amethod according to claim 19, wherein the pump is a centrifugal pump.21. A method according to claim 19, further comprising recovering thewellbore liquid to the surface.
 22. A method according to claim 19,further comprising transporting the wellbore liquid from a firstsubterranean location to a second subterranean location.
 23. A methodaccording to claim 19, wherein the pump is operated at more than 5,000rpm.
 24. A method according to claim 19, wherein the pump is operated at7,000 to 7,500 rpm.
 25. A method according to claim 20, wherein the pumpdraws the wellbore liquid from a plurality of lateral wellbores into acentral pump.
 26. A method of pumping wellbore liquid, comprising:installing an electric submersible pump, driven by an AC synchronouspermanent magnet motor, in a wellbore; and operating the pump at morethan 4,500 rpm to pump the wellbore liquid, wherein the motor comprises:a rotor, comprising: a central shaft; a plurality of tubular elementsspaced axially along the shaft; and a plurality of support rings spacedaxially along the shaft between the tubular elements and serving tosupport the tubular elements on the shaft so that the tubular elementsare rotatable with the shaft; a plurality of permanent magnets spacedabout a circumferential outer surface of each of the tubular elements; arespective retention sleeve tightly fitted over the permanent magnets oneach tubular element so as to retain the permanent magnets on thetubular element; and a stator coaxial with the rotor, comprising: astack of laminations; and radially spaced coils wound around the stack.27. A method of pumping wellbore liquid, comprising: installing anelectric submersible pump, driven by an AC synchronous permanent magnetmotor, in a wellbore; and operating the pump at more than 4,500 rpm topump the wellbore liquid, wherein the motor comprises: a rotor,comprising: a central shaft; and a carrier sleeve loosely fitted on theshaft; and rings closely engaging the shaft and supporting the carriersleeve; and a stator coaxial with the rotor, comprising: a stack oflaminations; and radially spaced coils wound around the stack.
 28. Amethod of pumping wellbore liquid, comprising: installing an electricsubmersible pump, driven by an AC synchronous permanent magnet motor, ina wellbore; and operating the pump at more than 4,500 rpm to pump thewellbore liquid, wherein the motor comprises: a rotor, comprising: acentral shaft; and a plurality of permanent magnets having axial ends; acarrier sleeve mounted on the shaft and bearing the magnets; a retentionsleeve extending over the magnets; and at least one stress-relievingradially outwardly extending abutment part on the carrier sleeveabutting an adjacent axial end of the magnets, wherein the retentionsleeve has at least one end portion turned in over the abutment part toretain the magnets in position on the carrier sleeve without damagingthe axial end of the magnet.
 29. A method of pumping wellbore liquid,comprising: installing an electric submersible pump, driven by an ACsynchronous permanent magnet motor, in a wellbore; and operating thepump at more than 4,500 rpm to pump the wellbore liquid, wherein themotor comprises: a rotor, comprising: an elongate central shaft; andelongate permanent magnets extending along the shaft, the magnetscomprising axially laminated parts to reduce eddy current losses; and astator coaxial with the rotor.
 30. A method of pumping wellbore liquid,comprising: installing an electric submersible pump, driven by an ACsynchronous permanent magnet motor, in a wellbore; and operating thepump at more than 4,500 rpm to pump the wellbore liquid, wherein: themotor comprises: a rotor; a stator coaxial with the rotor; a bearingmounting the rotor to the stator; a resiliently biased projectiondisposed on one of the stator and the bearing; and a receiver disposedon the other of the stator and the bearing, and the projection isoperable, by rotation of the rotor, to engage the receiver, therebyrotationally coupling the bearing and the stator.
 31. A method ofpumping wellbore liquid, comprising: installing an electric submersiblepump, driven by an AC synchronous permanent magnet motor, in a wellbore;and operating the pump at more than 4,500 rpm to pump the wellboreliquid, wherein the motor comprises: a rotor; a housing; a statorcoaxial with the rotor and mounted in the housing; an axial grooveformed in one of the stator and the housing; and an axial key engagingthe axial groove, thereby rotationally coupling the housing and thestator.